High voltage electrolytic capacitors are often employed in implantable medical devices. These capacitors are required to have a high energy density because it is desirable to minimize the overall size of the implanted device. This is particularly true of an implantable cardioverter defibrillator (“ICD”), also referred to as an implantable defibrillator, because the high voltage capacitors used to deliver the defibrillation pulse can occupy as much as one third of the ICD volume. ICDs typically use two to four electrolytic capacitors in series to achieve the desired high voltage for shock delivery. Typically, metal foils (e.g., aluminum foil) are used in the electrolytic capacitor due to their small size. Because the electrostatic capacitance of the capacitor is proportional to its electrode area, the surface of the metallic foil may be, prior to the formation of the dielectric film, roughened or subjected to a chemical conversion to increase its effective area. This step of roughening the surface of the metallic foil is called etching. Etching is normally carried out either by the method (chemical etching) of conducting immersion into a solution of hydrochloric acid or by the method (electrochemical etching) of carrying out electrolysis in an aqueous solution of hydrochloric acid. The capacitance of the electrolytic capacitor is determined by the extent of roughing (the surface area) of the anode foil and the thickness and the dielectric constant of the oxide film.
Due to the limited surface area that may be provided by etching metallic foils, attempts have also been made to employ porous sintered pellets in wet electrolytic capacitors—i.e., “wet tantalum” capacitors. A tantalum pellet, for instance, may be formed by compressing a powder under high pressure and sintering at high temperature to form a sponge-like structure, which is very strong and dense but also highly porous. As a result of the high voltages encountered in medical devices, however, low specific charge powders must generally be employed. Namely, if the specific charge is too high, relatively thin sinter necks tend to form between adjacent particles, which can cause the dielectric layer in the vicinity of these necks to fail at high voltages.
As such, a need currently exists for an improved wet electrolytic capacitor for use in implantable medical devices, such as defibrillators.